Catalytic cracking of heavy petroleum fractions is one of the major refining operations employed in the conversion of crude petroleum oils to desirable fuel products such as high-octane gasoline fuels used in spark-ignited internal combustion engines. Illustrative of "fluid" catalytic conversion processes is the fluid catalytic cracking process wherein suitably preheated high molecular weight hyrocarbon liquids and vapors are contacted with hot, finely-divided, solid catalyst particles, either in a fluidized bed reactor or in an elongated riser reactor, and maintained at an elevated temperature in a fluidized or dispersed state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons typically present in motor gasolines and distillate fuels. Suitable hydrocarbon feeds boil generally within the range from about 400.degree. to about 1200.degree. F. and are usually cracked at temperatures ranging from 850.degree. to 1200.degree. F.
In a catalytic process some non-volatile carbonaceous material, or "coke", is deposited on the catalyst particles. Coke comprises highly condensed aromatic hydrocarbons which generally contain 4-10 wt. % hydrogen. As coke builds up on the catalyst, the activity of the catalyst for cracking and the selectivity of the catalyst for producing gasoline blending stock diminish. The catalyst particles may recover a major proportion of their original capabilities by removal of most of the coke therefrom by a suitable regeneration process.
Catalyst regeneration is accomplished by burning the coke deposits from the catalyst surface with an oxygen-containing gas, such as air. Many regeneration techniques are practiced commercially whereby a significant restoration of catalyst activity is achieved in response to the degree of coke removal. As coke is progressively removed from the catalyst, removal of the remaining coke becomes most difficult and, in practice, an intermediate level of restored catalyst activity is accepted as an economic compromise.
The regenerated catalyst is recycled to contact fresh hydrocarbon feedstock and to convert hydrocarbons to more valuable products.